Below is a quick summary of everything showcased in the Video
Go to "Reports"
Click on "Create Report"
Click On "Audience Overlap"
Under "Fill In", input the two Instagram accounts separated by comma. For example: @fentybeauty,@jeffreestar and then click on "Confirm".
After you click on confirm, the profile picture of the Instagram handles will be displayed to you so that you can make sure that you've inserted the correct handles.
If everything is right, click "Confirm Report".
It can take up to 24 hours for an overlap analysis to be generated. You can check the status of the report under "My Reports".